Ships that transport goods around the world can carry nonindigenous (exotic) species in ballast water. The release of the ballast water from the ships is a major transport mechanism for the nonindigenous aquatic organisms (Carlton, 1985) as recognized by the U.S. National Invasive Species Control Act of 1996 (P.L. 104-332). Approximately 70,000 major cargo ships operating worldwide (Bureau of Transportation Statistics, 2008) pump ballast water on board to ensure stability and balance. Large vessels can carry in excess of 200,000 m3 of ballast, which is released in varying amounts at or when approaching cargo loading ports. In 1991, U.S. waters alone received approximately 57,000,000 metric tons of ballast water from foreign ports (Carlton et al., 1994). Ship surveys have demonstrated that ballast water is in general a non-selective transfer mechanism—many taxa representing planktonic and nectonic organisms capable of passing through coarse ballast water intake screens are common. These include bacteria, larval fish, zooplankton, and bloom forming dinoflagellates (Chu et al., 1997; Carlton and Geller, 1993).
The introduction of the nonindigenous (exotic) species has had dramatic negative effects on marine, estuarine, and freshwater ecosystems in the United States and abroad (Elton, 1958; Mooney and Drake, 1986; Chesapeake Bay Commission, 1995; NAS, 1996). Effects include alteration of the structure and dynamics of the ecosystem involved, including extirpation of native species (Office of Technology Assessment Archive, 1993).
The current state of the art for treating ballast water involves treating the water as it is pumped into or out of the ballast tanks. Methods for treating the water as it is pumped out the tanks are tremendously expensive and time consuming, and it is considered cost prohibitive to treat all water that is pumped into all tanks. The alternative to treating the water as it is pumped into or out of the tanks is to treat it while it resides in the tanks as the ship travels from port to port. To accomplish this, the entire volume of the tanks must be completely mixed in a relatively short time to ensure all the water in the tanks is exposed to the treatment method. This is especially true in emergency situations when a ship is grounded and the water in the ballast tanks must be treated before it is pumped out as part of the response plan to free the grounded vessel.
Methods for mixing water in tanks as part of a treatment process have been developed to treat waste water from municipal sewage systems, manufacturing, and industry. These treatment methods generally incorporate large circular or square tanks to hold the water during treatment, mixing, and neutralization (if required) before the water is released. These tanks generally lack geometric complexity and are therefore relatively easy to mix using a variety of mechanical methods (i.e. axial mixers, eductors, air, and nozzles). The ballast tanks on ships are quite different. The tanks are engineered to be part of the structure of the ship and are integral to the stability and integrity of the ship. As a result, most ships have multiple ballast tanks (ranging in number from a few to dozens) that are geometrically complex and often have baffles, support structures, web frames, stringers, stations, piping, and rose boxes inside the tanks. Also, there can be different types of ballast tanks with different geometries on a single ship. This complexity makes it difficult to mix the water in the tanks as part of a treatment method. Moreover, there are about 70,000 cargo ships operating worldwide. It would cost the shipping industry billions of dollars to install and maintain permanent mixing systems in all ballast tanks on all ships.